Hot air furnaces are well known having combustion chambers which provide a portion of the heat transfer and have secondary heat exchangers which provide for the remaining portion of heat exchange of the furnace. Quite often the heat exchanger is either of the drum or clam shell type, both of which take considerable room especially in the vertical dimension, and the latter of which normally incorporates the combustion chamber with the heat exchanger. Furthermore, these types of furnaces are complex and require expensive tooling, particularly when the clam shell type is considered.
As the cost of fuels have become more expensive, it has become increasingly important to increase the efficiency of a furnace. The traditional furnace with a combustor and a single heat exchanger normally has efficiency in the 70-75% range with a sizable portion of the usable heat being lost up the flue. This type of furnace does not utilize the latent heat of condensation obtained by condensing fluids from the combustion gases. Therefore, newer high efficiency furnaces have been designed to not only recover sensible heat, but also a portion of the latent heat of condensation, from the combustion gases prior to their being exhausted through the flue. These furnaces are sometimes referred to as recuperative or condensing furnaces and have an additional heat exchanger. While these furnaces can obtain efficiencies above 90%, they have been quite expensive. Also due to the additional elements, such as the extra heat exchangers, recuperative furnaces require complex designs which either cram more elements within the furnace cavity or require an increased sized furnace cavity in either the vertical or horizontal dimension. Since many furnaces are designed to be used with the central air conditioning, quite often with the air conditioning evaporator A-frame located above the furnace, increased vertical size of the furnace reduces the vertical space available between the top of the furnace and room structures. It would be desirable to reduce the standard furnace size so as to provide more vertical space for the air conditioning evaporator which in turn increases air conditioner efficiency.
It is also known to utilize fin and tube type heat exchangers, as the additional heat exchanger, in a recuperative furnace. Such heat exchangers have tubes made of stainless steel which resist corrosion from the combustion gases and the fluids condensed therefrom. However, it is desirable to utilize aluminum fins on such heat exchangers due to the high heat transfer coefficient of aluminum and its relatively low cost when compared to other high heat transfer materials such as copper. However, the aluminum fins cannot be subjected to excessively high temperatures due to the fin material itself and more importantly due to the relative temperature coefficients of expansion between the aluminum fin and the tube materials, such as stainless steel which causes a breach of the thermal conductivity between the aluminum fins and the tube material, if raised to excessive temperatures.